This invention relates generally to laser interferometer systems and more specifically relates to a system for altering the digital distance display in a Zeeman-effect laser interferometer system.
In recent years it has become increasingly important to be able to provided dimensional inspection of machined parts to within accuracies of .+-..0001 inch due to increased accuracies in tolerance requirements in precision machined parts. Normally, this dimensional inspection is done using a three-axis machine similar to the machines on which the part is cut. In order to increase the accuracy of dimensional measurements, the machines are normally equipped with laser interferometers to accurately position gauge heads and subsequently provide dimensional readouts in the gauging operation for dimensional inspection.
One particular type of interferometer system which is used in dimensional inspection is a Zeeman-type laser interferometer system. This type of interferometer system generates a distance measurement by selectively interfering two light frequencies produced by Zeeman splitting in making a comparison of a Doppler light signal reflected from a moving axis target with a reference light signal. When the target is at rest the comparison produces output pulses from photodetectors viewing the separate fringe patterns at the same frequency. When the target moves, the fringe patterns produced by the Doppler light beam will be faster or slower than the reference fringe patterns depending upon the direcion of target movement. This system is currently the most popular interferometer system available to make precision measurements. A typical interferometer of this type is the Model No. 5525B available from Hewlett-Packard, 1101 Embarcadero Road, Palo Alto, California, 94303. Details of the laser system may be had by referring to the operator's manual for the above-referenced interferometer.
One of the problems in using this type of interferometer system is that it is very difficult to initially position a tool or gauging transducer along an axis to be cut or measured relative to a machine reference datum. This problem can be eliminated by simply presetting the digital display unit interferometer to read out the desired reference distance from the datum point. Unfortunately, the electronics required to manipulate the digital display of a Zeeman-effect interferometer are very complex. This is due to the fact that any display unit for a Zeeman-effect interferometer must actually calculate each measurement, considering such variables as barometric pressure, temperature, and humidity in each calculation. Since a digital display unit of this type is so difficult to modify, a preset system is presently not available for a Zeeman-effect interferometer system. It has been the practice in the past to orient a part on a dimensional inspection machine and manually measure the distance from a reference datum, such as a bed on a dimensional inspection table of the machine, to the reference point on a particular part to be checked. This method requires that the operator subtract this distance from each dimensional measurement along that particular axis for each inspection point. This method is not only time consuming but may introduce costly errors in part qualification and inspection.
Therefore, there is a need for a system which may be readily connected to a Zeeman-effect interferometer system to conveniently preset the digital display when a gauge or tool is set at the machine datum point along an axis so that when the reference point of the part along that axis is reached by the gauge head the digital display will read zero and each measurement thereafter is the actual measurement from the part reference point along the particular axis as the part is inspected.